Polyolefins are plastic materials useful for making a wide variety of valued products due to their combination of stiffness, ductility, barrier properties, temperature resistance, optical properties, availability, and low cost.
The use of terpene and hydrogenated hydrocarbon resins as modifiers for polypropylene (PP) converted into oriented film is well known. Some of the attributes assigned to the use of low molecular weight resin products in polypropylene films, include good optical properties, improved processing when making oriented films, better sealing characteristics, and desirable mechanical properties and converting characteristics.
The use of hydrocarbon resins (HCR) for improving the moisture barrier properties of oriented polypropylene is also well known. The effectiveness of resin for improving barrier properties is expected to be highly dependent on the characteristics of the PP itself. These characteristics include the degree of crystallinity of the PP, the compatibility of the resin with the polypropylene amorphous regions and the amorphous region's glass transition.
Additionally, it has been generally known that high levels of hydrocarbon resin were required to cause substantial improvements in barrier properties of polypropylene film, typically in the range 5% to 25% by weight. However, adding resin at these levels typically embrittles non-oriented PP film to an excessive degree. In oriented polypropylene (OPP) film, the orientation imparted to the polymer offsets the negative effect of the resin on ductility, so that films with good mechanical properties can be produced at the high loadings of hydrocarbon resin required to impart improvements in barrier properties.
Because of differences between ethylene polymers and polypropylene in crystallinity level, glass transition temperature, and amorphous character (linear vs. branched aliphatic structure), the effects of hydrocarbon resins in polyethylene films can not be strictly predicted based on analogy with oriented polypropylene films. Additionally, because most polyethylene films possess a relatively low degree of molecular orientation as compared to OPP films, the ability to incorporate hydrocarbon resins in polyethylene films at an effective level without ruining mechanical properties is an area of concern.
High density polyethylene (HDPE) is nominally a linear homopolymer of ethylene containing few branch points in the polymer chain. As a result of its regular structure, HDPE is a highly crystalline material with a peak crystalline melting point typically around 135° C. Various types of HDPE are characterized by the density of the material, which ranges typically from 0.940 to 0.965 (g/cc). Density is a measure of the crystallinity developed by the HDPE material, where higher density relates to higher the level of crystallinity developed by the polymer. Mechanical properties and barrier properties are strongly influenced by the degree of crystallinity developed in the HDPE polymer.
Typical uses are in the production of blow molded containers such as milk bottles, molded articles, lightweight consumer bags and trash bags, and various types of film products.
One example of a HDPE film product is the inside liner used to package cereal products. In this and similar packaging applications, superior barrier properties of the HDPE, relative to non-oriented PP or low density PE films, is a very positive attribute of the HDPE film. One type of barrier property refers to preventing the permeation of moisture either in or out of the packaged food product.
The need exists for a method for a method for the incorporation of various hydrocarbon resins into high density polyethylene polymers (HDPE). The need also exists for films which possess superior barrier properties and still retain desirable mechanical properties such that the films can be used for packaging film applications where improved barrier properties of these films have value. Additionally, the need exists for a highly efficient process for producing films of HDPE modified with hydrocarbon resin. It has been found that by adding various types of hydrocarbon resins to HDPE polymer to form a blend, and forming a film from the blend, a superior packaging film can be produced with improved moisture barrier properties than films produced from the HDPE polymer by itself. These improved barrier films have value in packaging applications where a reduced rate of moisture loss (or gain) increases the shelf life of the packaged material. In the opposite sense, by improving the barrier properties of the HDPE film the thickness of the film used to package a material may be reduced, lowering the amount of packaging material required, and, as a result, reducing the amount of refuse derived from packaging film.